As described in, for example, U.S. Pat. No. 3,767,056, with an injection molding of plastic material in an injection molding apparatus, it is desired to filter hot running plastic to remove impurities or foreign materials, bits of unmelted plastic and the like during an injection step where the hot plastic material is forced to run toward a mold cavity. Otherwise, clogging of a melt passage leading to a mold cavity with such impurities is likely to occur with the result that a long run operation of injection molding is interrupted. This interruption may require replacement of a hot or cold runner mold and/or a manifold forming a section of the melt passage, and such replacement leads to an increased equipment or machine cost, and also leads to a reduction in molding productivity since the continuously repeated injection molding operation is interrupted.
In this regard, prior arts including the above U.S. patent incorporate melt filtering devices provided in a nozzle detachably connected to a barrel forming a body of the injection machine. The filtering device comprises a tubular member having a plurality of round or elongated perforations formed therethrough in a parallel and spaced relation The perforated tubular member is disposed in or connected to the nozzle so that the perforations communicate with the nozzle.
According to some of the prior arts, such a melt filtering device or means is provided with a means for purging the impurities accumulated upstream of, but in the vicinity of, the perforations, as needed. Such a purging operation, for example, is performed with the melt including the accumulated impurities discharged out of an outlet of the nozzle without passing through the perforations. This requires detachment of the nozzle with the barrel from a mold arrangement defining the mold cavity, and thus requires a relatively long period of time to reset up the injection molding machine for a normal injection molding operation.
In the meantime, it is noted that the conventional melt filtering device has an inherent problem in that it is subjected to a high pressure loss or exhibits a high pressure resistance against a hot plasticized material, the so called "melt", in the process of injection. Such a high pressure loss due to the filtration is increased as filtration performance is increased and also as an injection rate or speed is increased. The filtration performance relies on a size of the perforations.
With a nozzle provided with no such melt filtering device, an injection molding apparatus comprising a mold arrangement, including a cavity mold and a hot runner mold and/or a manifold, and an injection machine having the nozzle, is required to be subjected to a high fluid pressure loss or exert an injection pressure, for example, of 100 kg/cm.sup.2 to attain a desired injection rate. In this connection, the same injection molding apparatus but with a melt filtering device provided in the nozzle is required to exert a higher injection pressure of 115 or more kg/cm.sup.2 to attain the same injection rate. The increase in the injection pressure is due to the flow resistance of the melt filtering device, and with a higher injection rate an increased injection power is required accordingly. The higher injection power, naturally, requires an increased driving power to be exerted by the injection machine. This results in that some cases require a more powerful hydraulic driving apparatus to be incorporated in the injection machine for driving a screw plunger disposed in the barrel for rotation and axial reciprocation, resulting in increased machine cost.
Otherwise, or in a less powerful machine case, an apparatus with melt filtering means is likely to operate with an injection rate lower (for example, 60 to 65 g/sec) than that (for example, 70 g/sec) of the same apparatus but with melt filtering nozzle means provided. Such a decreased injection rate in the non-filtering apparatus affects production of a high quality molded article, particularly a thin product having a complicated contour.
Apart from the above, there is a problem in that it is not easy to provide a molded product, which is of a high quality due to a uniform distribution of different plastic materials and/or coloring pigments or the like therein. To solve the problem, there have been various efforts made in designing a screw plunger and providing starting materials which consist of a plurality of materials well mixed with each other in advance for enhancing the distribution degree of the materials in a molded product.